In a conventional internal combustion engine, it is important to reduce friction between an inner surface of a cylinder bore and a piston ring to enhance fuel economy. Especially in the piston ring of the internal combustion engine, it is required to enhance qualities of a pressure ring and an oil ring to reduce a frictional force, and it is also required to reduce oil consumption, as well as the frictional force.
The oil ring has a function which allows lubricating oil to infiltrate into a cylinder bore inner surface to form an appropriate oil film, and to scrape excessive oil on the cylinder bore inner surface, and an oil-control function which controls consumption of the lubricating oil. To enhance these functions of the oil ring, a technique for reducing the axial direction width of the oil ring has been developed.
As shown in FIG. 1B, an oil ring body 1 has two rails 10 and 11 which are connected to each other through a columnar web 15. The oil ring body 1 has a substantially I-shaped cross section. Oil scraped from an inner wall of a cylinder by a sliding surface 12 passes through many oil-passing holes 2 formed in the web 15 and moves toward an inner periphery of the oil ring.
In the oil ring having such a structure, in order to reduce the lubricating oil consumption, the axial direction width of the oil ring, i.e., the height of h1 shown in FIG. 1A is reduced, thereby enhancing the oil control function of the oil ring. However, if the oil ring body is reduced in width, since an area of the oil-passing hole can not be maintained broadly enough, the scraped oil can not swiftly be discharged into the oil ring, and it is difficult to secure the area of the oil-passing hole which is required for smoothly returning the oil into an oil pan. If the area of the oil-passing hole is increased, a thin oil ring has a large opening, and there is a problem that an abutment portion is bent and broken when the oil ring is assembled.
In Patent Document 1, for example, to enhance the strength of the oil ring, a technique for forming a shape of the oil-passing hole into an elliptic shape or a diamond shape is disclosed. According to this technique, by forming the oil-passing hole into the elliptic shape or diamond shape, variation in second moment of area around a periphery of an edge of the oil-passing hole can be moderated, and swelling generated when the oil ring is bent can be reduced.
However, in order to secure the area of the oil-passing hole, it is preferable that the shape of the oil-passing hole in its longitudinal direction is close to a straight line which is in parallel to a radial direction of the oil ring, and the oil-passing hole having the elliptic shape or diamond shape is not preferable for securing the oil passing area.
Patent Document 2 proposes a three piece-type combination oil ring including upper and lower side rails and a spacer expander. In order to prevent a stress concentration at both circumferential direction ends of the oil-passing hole, in the spacer expander portion, the oil ring of the combination oil ring has a long oil-passing hole whose both ends are formed into arc shape. With this shape, it is possible to secure a wide area of the oil-passing hole. Such an oil ring is applicable for the three piece-type expander, but when this is applied to an oil ring in which the upper and lower rails are integrally formed, since the arc portions become large, a cross sectional area of the supporting ring main body becomes small, and there is a problem that a breaking resistance is insufficient.
Patent Document 1: Japanese Patent Application Laid-Open No. 63-277844
Patent Document 2: Japanese Patent Application Laid-Open No. 2001-32934